Piston Cooling Configuration Utilizing Lubricating Oil From Bearing Reservoir In An Opposed-Piston Engine

ABSTRACT

Pressurized lubricating oil is accumulated in the bearings of opposed pistons and accumulated oil is dispensed therefrom for bearing lubrication and also for cooling the undercrowns of the pistons by jets of oil emitted from the bearings.

RELATED APPLICATIONS

This application contains subject matter related to the subject matterof the Applicant's U.S. patent application Ser. No. 13/136,955, filedAug. 15, 2011 for “Piston Constructions for Opposed-Piston Engines,”published as US 2012/0073526 on Mar. 29, 2012, U.S. patent applicationSer. No. 13/776,656, filed Feb. 25, 2013 for “Rocking Journal Bearingsfor Two-Stroke Cycle Engines”, and U.S. patent application Ser. No.14/075,926, filed Nov. 22, 2013 for “Lubricating Configuration ForMaintaining Wristpin Oil Pressure In A Two-Stroke Cycle, Opposed-PistonEngine”.

BACKGROUND

The field is piston thermal management for internal combustion engines.More specifically the application relates to implementation of a pistoncooling configuration for an opposed-piston engine in which theundercrown—that portion of the piston crown that is behind or underneaththe crown end surface against which combustion acts—is cooled by use ofone or more jets of lubricating oil fed from a reservoir in the piston'sbearing mechanism.

Piston thermal management presents continuing challenges to pistonintegrity due to increasing loads demanded for modern engines. In atypical piston, four areas are particularly susceptible to thermaldamage: the piston crown, the ring grooves, the piston/wristpininterface, and the piston undercrown. If combustion temperatures felt bythe crown end surface exceed the oxidation temperature of the crownmaterials, oxidation can result. The crown may be subject to mechanicalfailure caused by stress/fatigue at the oxidized sites. The piston'srings, ring grooves, and lands may exhibit carbon build-up due tolubricating oil being heated above its coking temperature. A hotwristpin bore can result in lower load-carrying capacity of the pistonbearing. As with the ring grooves, the piston undercrown may also besubject to oil coking.

In some aspects of opposed-piston combustion chamber construction it isdesirable to utilize pistons whose crowns include highly contoured endsurfaces which produce complex, turbulent charge air motion thatencourages uniform mixing of air and fuel. An example of a highlycontoured piston end surface that forms a combustion chamber with anoppositely-disposed, similarly-contoured piston end surface is shown inFIG. 11 of the Applicant's US 2011/0271932 A1. Combustion imposes aheavy thermal load on these pistons. Their highly contoured end surfacescreate non-uniform thermal profiles with concentrations of heat (“hotspots”) that can lead to asymmetrical thermal stress, wear, and pistonfracture.

Typically, three approaches are taken to manage piston temperatures. Inone, high thermal resistance of the piston crown reduces or blocks thepassage of heat from the combustion chamber into the crown. A secondapproach relies on conduction of heat from the crown to the cylinderbore through the rings, ring grooves, lands, and skirt of the piston.The third approach uses a flow of liquid coolant to remove heat from theundercrown. Modern piston constructions typically include all threeapproaches.

Liquid coolant is typically applied to the undercrown by means ofgalleries and/or nozzles. For example, U.S. Pat. No. 8,430,070 teaches apiston cooling construction including an outer gallery that receives andtransports oil for cooling the piston undercrown. An oil outlet isprovided on the bottom of the outer gallery. A nozzle mounted to thefloor of the outer gallery, in fluid communication with the oil outletis aimed toward the undercrown. Oil is inertially pumped from thegallery through the oil outlet in response to upward movement of thepiston. The pumped oil is sprayed from the nozzle onto the undercrown inresponse to upward movement of the piston.

An example of undercrown cooling in an opposed-piston context is shownin FIG. 5 of the Applicant's US 2012/0073526 A1 wherein a piston with acontoured end surface includes an annular gallery 256 within the crownthat follows the periphery of the crown, underneath the end surface. Theannular gallery is in fluid communication with a central gallery 257underneath the central portion of the end surface. A nozzle 262,separate from the piston, is aimed at an opening in the annular gallery256. A high velocity jet of oil emitted by the nozzle 262 travels intothe annular gallery, striking a specific portion of the crown underneatha ridge of the end surface that bears a heavy thermal burden duringcombustion. The jet cools the specific crown portion by impingement. Theoil then flows through the annular and central galleries, therebycooling additional portions of the undercrown. Oil flows out of thecentral gallery and exits the piston.

The cooling capability of the nozzle described in U.S. Pat. No.8,430,070 is limited by the inertial pumping operation which occurs onlyduring upward movement of the piston. As a result, the undercrown iscooled by spraying oil through only one half of the piston's operationalcycle. Furthermore, because the sprayed oil is obtained from the coolinggallery, it is already heated, which limits its cooling capacity whenemitted by the nozzle. The cooling construction of US 2012/0073526 A1brings oil into the piston via a nozzle external to the piston. Separatetransport channels are required to bring up pressurized oil to cool theundercrown and to lubricate the piston rod coupling mechanism. As aresult, oil is provided throughout the operating cycle of the piston,but at the penalty of increased complexity and cost of the lubricationsystem.

Accordingly, there is a need for delivering lubricating oil to a pistonfor cooling the undercrown in a manner that maintains the flow oflubricating oil throughout the piston's cycle of operation withoutadding to the complexity and cost of the system that transports the oilto the piston for lubrication.

SUMMARY

In order to cool the undercrown of a piston with pressurized lubricatingoil throughout the piston's cycle of operation, without adding to thecomplexity and cost of the system that transports the oil to the pistonfor lubrication, oil is actively pumped to a reservoir in the piston'sbearing for lubricating the bearing. From the reservoir, the pressurizedoil is also provided to one or more cooling jet outlets provided in thebearing and aimed at the undercrown.

In some aspects, the reservoir is in the wristpin of the piston bearing.In some further aspects, the reservoir is in a piston bearing wristpinattached to the small end of a connecting rod.

In some aspects, a stationary cooling jet outlet in fluid communicationwith a bearing oil reservoir is positioned to emit a jet of oil targetedat a specific, large region of a piston undercrown. The stationarycooling jet outlet may be disposed in a bearing part that does not moverelative to the undercrown. In some aspects, the bearing part supportsthe wristpin for oscillating movement with respect to the undercrownduring engine operation.

In some aspects, a cooling jet outlet includes a movable nozzle in fluidcommunication with a bearing oil reservoir. The nozzle is positioned toemit a jet of oil targeted to a specific, large region of a pistonundercrown. The nozzle may be mounted to a piston part that movesrelative to the undercrown as the piston travels so as to sweep theregion with the jet. In some other aspects, the part oscillates withinthe piston skirt in response to piston movement so that the jetcontinuously sweeps the region with each cycle of piston movement. Inyet other aspects, the nozzle is mounted to an element of the connectingrod that oscillates, or rocks, with respect to the undercrown duringengine operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematic drawing of a prior art opposed-piston engine with apump-supplied oil gallery.

FIG. 2 is a side view, in perspective, of a piston/connecting rodassembly for a two-stroke cycle, opposed-piston engine in which thepiston has a first end surface construction.

FIG. 3 is a cross-sectional view of the piston crown and skirt takenalong lines A-A of FIG. 2.

FIG. 4 is an exploded view of the piston/connecting rod assembly of FIG.2 showing elements of a bearing configuration.

FIG. 5A is a cross-sectional view of the piston/connecting rod assemblytaken along lines A-A of FIG. 2 showing a first cooling jet outletconstruction.

FIG. 5B is a cross-sectional view of the piston/connecting rod assemblytaken along lines B-B of FIG. 2 showing a second cooling jet outletconstruction.

FIG. 6 is a cross-sectional view of the crown and skirt of a pistonhaving a second end surface construction.

FIG. 7A is a cross-sectional view showing the piston of FIG. 6 assembledto a bearing with a variation of the first cooling jet outletconstruction.

FIG. 7B is a cross-sectional view showing the piston of FIG. 6Aassembled to a bearing with a variation of the second cooling jet outletconstruction.

DETAILED DESCRIPTION

A two-stroke cycle engine is an internal combustion engine thatcompletes a power cycle with a single complete rotation of a crankshaftand two strokes of a piston connected to the crankshaft. One example ofa two-stroke cycle engine is an opposed-piston engine in which a pair ofpistons is disposed in opposition in the bore of a cylinder. Duringengine operation, combustion takes place in a combustion chamber formedbetween the end surfaces of the pistons.

As seen in FIG. 1, an opposed-piston engine 49 has at least one portedcylinder 50. For example, the engine may have one ported cylinder, twoported cylinders, three ported cylinders, or four or more portedcylinders. For purposes of illustration, the engine 49 is presumed tohave a plurality of ported cylinders. Each cylinder 50 has a bore 52.Exhaust and intake ports 54 and 56 are formed in respective ends of thecylinder such that the exhaust port 54 is longitudinally separated fromthe intake port 56. Each of the exhaust and intake ports 54 and 56includes one or more circumferential arrays of openings. Exhaust andintake pistons 60 and 62 are slidably disposed in the bore 52 with theirend surfaces 61 and 63 opposing one another. The exhaust pistons 60 arecoupled to a crankshaft 71, and the intake pistons are coupled to acrankshaft 72. Each of the pistons is coupled to its associatedcrankshaft by a bearing 74 and a connecting rod 76.

A lubrication system that supplies oil to lubricate the moving parts ofthe engine 49 includes an oil reservoir 80 from which pressurized oil ispumped by a pump 82 to a main gallery 84. The main gallery suppliespressurized oil to the crankshafts 71 and 72, typically throughdrillings 86 to the main bearings (not seen). From grooves in the mainbearings, pressurized oil is provided to grooves in the big end bearingsof the connecting rods 76. From there, pressurized oil flows throughdrillings 77 in the connecting rods to the bearings 74.

In some aspects, which are not intended to be limiting, the engine 49 isequipped with an air management system 51 that includes a supercharger110 and a turbocharger 120. The turbocharger has a turbine 121 and acompressor 122 rotating on a common shaft 123. The turbine 121 iscoupled to the exhaust subsystem and the compressor 122 is coupled tothe charge air subsystem. Exhaust gas emptied into the conduit 125 fromthe exhaust port 54 rotate the turbine 121. This rotates the compressor122, causing it to generate charge air by compressing intake air. Thecharge air output by the compressor 122 flows through a conduit 126,whence it is pumped by the supercharger 110 to the openings of theintake port 56.

The operational cycle of an opposed-piston engine is well understood. Inresponse to combustion occurring between their end surfaces 61, 63, theopposed pistons 60, 62 move away from respective top center (TC)locations in the cylinder. While moving from TC, the pistons keep theirassociated ports closed until they approach respective bottom center(BC) positions. The pistons may move in phase so that the exhaust andintake ports 54, 56 open and close in unison; alternatively, one pistonmay lead the other in phase, in which case the intake and exhaust portshave different opening and closing times. As the pistons move throughtheir BC locations exhaust products flowing out of the exhaust port 54are replaced by charge air flowing into the cylinder through the intakeport 56. After reaching BC, the pistons reverse direction and the portsare again closed by the pistons. While the pistons continue movingtoward TC, the charge air in the cylinder 50 is compressed between theend surfaces 61 and 63. As the pistons advance to their respective TClocations in the cylinder bore, fuel is injected through the nozzles 100into the charge air, and the mixture of charge air and fuel iscompressed in the combustion chamber formed between the end surfaces 61and 63 of the pistons 60 and 62. When the mixture reaches an ignitiontemperature, the fuel ignites. Combustion results, driving the pistonsapart, toward their respective BC locations.

In some cases, the opposing end surfaces 61 and 63 are identicallyconstructed and the pistons 60 and 62 are disposed in rotationalopposition with reference to the axis of the cylinder in which they aredisposed. See, for example, the piston end surface constructionsdescribed and illustrated in the Applicant's US publication 2011/0271932A1 and US publication 2013/0213342 A1. In some other cases, the opposingend surfaces 61 and 63 have complementary constructions which do notrequire rotational opposition. See, for example, the piston end surfaceconstructions described and illustrated in the Applicant's WOpublication 2012/158756 A1 and related U.S. application Ser. No.14/026,931.

It is desirable to include undercrown cooling in the thermal design ofthe pistons of an opposed-piston engine such as the engine 49 shown inFIG. 1. Therefore, the undercrown cooling embodiments described andillustrated in this specification may be combined with other modes ofpiston thermal management in order to realize an effective pistonthermal performance. Undercrown cooling and other objectives areachieved by accumulating pressurized lubricating oil in the pistonbearings of an opposed-piston engine and dispensing accumulated oiltherefrom for bearing lubrication and also for cooling the undercrownsof the pistons by way of one or more jets.

First End Surface Construction:

FIG. 2 is a perspective view of a piston assembly used in anopposed-piston engine in which the pistons have identical end surfaces.In this case the description is directed to a single piston, with theunderstanding that it applies as well to its opposing counterpart. Thepiston assembly includes a piston 200 and its associated connecting rod210. The piston 200 has a crown 203, and a skirt 205. An end surface 206of the crown is configured to form a combustion chamber in cooperationwith the end surface of an identically-configured, opposing piston.Lands and ring grooves 207 are provided in the crown's side wall.Referring to FIGS. 2, 3, and 4, the piston includes an undercrown 208,which is that portion of the piston crown 203 that is behind orunderneath the end surface 206 against which combustion acts. Theconnecting rod 210 has a large end 211 for coupling to a crank throw ofa crankshaft (not seen). An oil groove 212 is formed in the bearingsurface of the large end 211. An oil delivery passage 213 (best seen inFIG. 5B) extends longitudinally in the connecting rod 210 from the oilgroove 212 to the small end 215.

As per FIGS. 2 and 3, the contour of the end surface 206 includes aridge 209 formed by complex curved surfaces that interact with bulk airmotion components to enhance charge air turbulence in the combustionchamber. During combustion, a hot spot occurs in the portion of thecrown including the ridge 209.

Piston Bearing Construction and Lubrication:

With reference to FIGS. 4, 5A, and 5B, the piston 200 includes a bearing217 fixed to the undercrown 208 and disposed in space encircled by theskirt 205. As per FIGS. 4 and 5B, a bearing structure includes bearingsupport members 219 a and 219 b. The bearing support member 219 aincludes a bearing surface 220 that receives a wristpin 221 (also calleda “journal” or a “gudgeon pin”) mounted to the small end 215 by threadedfasteners. In some instances, the bearing support members 219 a and 219b are joined around the wristpin 221 and secured to the undercrown byfasteners 218 that are threadably seated in interior structures of theundercrown 208. As seen in FIG. 4, an opening 223 in the bearing supportmember 219 b receives the connecting rod 210. When assembled, thebearing support member 219 a, 219 b retains the wristpin 221 foroscillation on the bearing surface 220 caused by arcuate oscillation ofthe connecting rod 210 in the opening 223.

As per FIGS. 4 and 5B, the wristpin 221 includes an internal oilreservoir 222 in fluid communication with an oil inlet passage 224drilled through the wristpin 221 and one or more oil outlet passages 225drilled through the wristpin 221. Circumferential oiling grooves 227 areformed in the bearing surface 220 in alignment and fluid communicationwith the oil outlet passages 225. In some aspects, best seen in FIG. 4,the oil reservoir 222 can be configured as an annular recess withopposing ends and an axis that corresponds to the axis on which thewristpin 221 oscillates. In this case, a cylindrical shaft 250 isreceived in the cylindrical inner surface 228 of the wristpin 221. Aflange 252 is fixed to one end of the shaft 250; the opposite end of theshaft is threaded to another flange 254 so as to retain the shaft 250 incoaxial alignment with the cylindrical inner surface 228 of the wristpin221. As the views in FIGS. 4 and 5B show, the smaller diameter of theshaft 252 results in the formation of an annular space between itselfand the inner surface 228. In other instances the oil reservoir may beformed by a capped or stoppered drilling in the wristpin 221.

With reference to FIG. 5B, pressurized oil is transported to the largeend oil groove 212 in the manner illustrated by the transport path 80,82, 84, 86 in FIG. 1. From the oil groove 212, pressurized oil isdelivered to the oil reservoir 222 via the oil reservoir inlet passage224. Pressurized oil received in the oil reservoir 222 is provided inmultiple streams through the outlet passages 225 to the oiling grooves227 whence it flows to lubricate the bearing interface between thebearing surface 220 and the wristpin 221.

In some aspects, but not necessarily, the bearing 217 may be constructedas a rocking journal bearing (also called a “biaxial” bearing). Such abearing is described in the Applicant's U.S. Ser. No. 13/776,656. Inthis case the bearing surface 220 comprises a plurality ofaxially-spaced, eccentrically-disposed surface segments and the wristpin221 includes a corresponding plurality of axially-spaced,eccentrically-disposed wristpin segments. In such cases, the bearingsurface 220 may have a semi-cylindrical configuration with two lateralsurface segments sharing a first centerline and a central surfacesegment separating the two lateral surface segments and having a secondcenterline offset from the first centerline. In such cases, thecircumferential oiling grooves 227 are formed in the bearing surface 220at the borders between the central surface segment and the lateralsurface segments. In some instances, the outer surface of the wristpin221 may have axially-spaced circumferential grooves 226 (best seen inFIG. 4) and one or more circumferentially-spaced axial oiling grooves(not seen) may also be formed in the bearing surface 220 to enhance thelubrication of the bearing interface.

Gallery Cooling:

As best seen in FIGS. 3, 4 and 5A, the undercrown 208 is cooled by flowof lubricating oil through one or more galleries internal to the crown203. Preferably, the galleries comprise shaped spaces in the undercrown208 with floors provided on the upper surface of the bearing supportmember 219 a. For example, an annular gallery 256 follows the peripheryof the crown 203 and girds a central gallery 257. The annular gallery256 communicates with the central gallery 257 through holes 258. Theannular gallery 256 has an asymmetric profile that rises as at 260 underthe ridge 209. The central gallery 257 abuts the deepest part of the endsurface 206. Lubricating oil for cooling the undercrown is provided tothe galleries via a high velocity jet of pressurized oil. The jetstrikes the interior surface of the annular gallery at its highest point260, thereby cooling that portion of the undercrown 208 beneath theridge 209 by impingement. The oil carried by the jet flows from therethroughout the annular gallery 256. From the annular gallery 256, thelubricating oil flows into the central gallery 257 where it continuouslyirrigates the central portion of the undercrown along the inside of theridge 209. Lubricating oil flowing throughout the annular gallery 256washes and cools an annular portion of the undercrown 208 that abuts thelands and ring grooves 207. Reciprocation of the piston agitates theoil, which causes it to circulate in the annular gallery 256. Inresponse to the agitation and piston motion, oil also flows from theannular gallery 256 into the central gallery 257. Piston reciprocationalso agitates the oil collected or accumulated in the central gallery257 so as to cool the undercrown 208 beneath the central portion of theend surface 206. The oil flows from the bottom of central gallery 257via one or more passages in the bearing structure into the interior ofthe skirt 205. From there, the oil flows out the open end of the skirt205.

In the prior art gallery cooling constructions described in theApplicant's US 2012/0073526 A1, the lubricating oil jets for cooling theundercrown are provided to the piston galleries from nozzles that areseparate from, external to, and fixed with respect to, the pistons. Inthe embodiments to be described, the oil jets are delivered fromelements of the pistons themselves and are fed from oil reservoirs inthe piston bearings.

Cooling Jet Constructions:

With regard to the piston lubrication constructions thus far described,the pressurized oil delivered to a bearing oil reservoir for lubricationmay at the same time be used for undercrown cooling in an opposed-pistonengine. In some aspects, pressurized oil obtained from a bearing oilreservoir is provided in the form of a high velocity stream or jet forcooling a piston undercrown. Hereinafter such a jet is referred to as a“cooling jet”, for convenience and clarity. A cooling jet is providedfrom a cooling jet outlet that is in fluid communication with thebearing oil reservoir. At least one cooling jet constituted of received,pressurized lubricating oil is provided from each piston bearing so asto cool a portion of the undercrown by impingement. Jetted oil flowsfrom the undercrown portion into the piston cooling galleries so as toprovide a constant replenishment of coolant with which to cool the restof the undercrown by irrigation. A cooling jet may be stationary, or itmay be swept in an oscillating motion.

FIGS. 4, 5A, and 5B illustrate two cooling jet embodiments. Althoughboth embodiments are shown together in one or more of the figures, thisis for convenience and clarity. In fact the embodiments are proposed asalternatives. That is to say a piston may be equipped with one or theother, as required by design considerations and cost.

First Embodiment

Continuing with the exemplary piston construction shown in FIGS. 4 and5A, a first cooling jet outlet embodiment is constituted of a passage270 drilled or formed in the bearing support member 219 a. One end ofthe passage 270 is in fluid communication with the oil reservoir 222 viaan oiling groove 227. When pressurized oil received in the reservoirenters the oiling grooves 227, a portion of it flows into the passage270 whence it is emitted in the form of a coolant jet 272. The passage270 is located and oriented so as to be aimed at the undercrown portion260. As a result, the coolant jet 272 is directed toward the hot spot inthe crown 203 that is associated with the ridge 209. As should beevident with regard to the figures, the bearing support member 219 a isfixed to the undercrown. Lacking relative movement between the member219 a and the undercrown 208, the jet 272 is stationary with respect tothe undercrown 208.

Second Embodiment

An alternative coolant jet outlet is best seen in FIGS. 4 and 5B, wherea nozzle 278 is mounted to the wristpin 221 so as to be in fluidcommunication with the reservoir 222 by way of an oil outlet 280. In theillustrated example, the oil outlet 280 is formed in the flange 252, andthe nozzle 278 is mounted thereto by an elbow joint 281 received in theoil outlet. The nozzle 278 extends into the annular gallery 256, aimedtoward the undercrown portion 260. Pressurized oil received in thereservoir 222 flows into the nozzle through the oil outlet 280 and theelbow joint 281. From the nozzle 278, the pressurized oil is emitted inthe form of a coolant jet 284. As should be evident with regard to thefigures, the wristpin 221 undergoes oscillatory movement relative to theundercrown 208. As a result, the coolant jet 284 is swept to and froacross the undercrown portion 260.

Second Piston Construction:

In some aspects, it may be desirable to provide more than one coolingjet to the undercrown of a piston with an end surface constructionhaving a more complex contour than that of the piston of FIG. 2. Forexample, the first combustion chamber construction for an opposed-pistonengine described and illustrated in the Applicant's WO2012/158756includes a piston end surface with two opposing ridges, each with anassociated hot spot. Such a piston is shown in FIG. 6, in which thepiston 300 has a crown 303 with an end surface 306 and an undercrown308. The end surface includes an elongated cleft 307 extending in adiametrical direction of the piston 300, defined between opposing ridges309. As best seen in FIGS. 7A and 78, the undercrown 308 includesannular and central cooling galleries and the piston includes a bearingstructure identical to that of the bearing 217, except that the firstand second cooling jet embodiments include two jet outlets each, witheach cooling jet outlet aimed at a respective hot spot portion of theundercrown.

Method of Operating an Opposed-Piston Engine:

With reference to the figures, an opposed-piston engine such as theengine 49 includes at least one cylinder 50 and a pair of pistons 200equipped with bearing constructions as described herein. The pistons aredisposed in opposition to one another in a bore 52 of the cylinder, andeach piston is connected to a respective connecting rod 210 by a bearing217. The engine is operated by a method that includes providing a flowof pressurized oil to each bearing 217. The flow of pressurized oil toeach bearing 217 is received in a wristpin 222 of the bearing. Inresponse to pressurized oil in the wristpins, multiple streams of thereceived, pressurized oil from each wristpin are provided to lubricate arespective bearing interface, and at least one jet of the received,pressurized oil is provided from one of a fixed part and a moving partof each bearing, in which each jet is aimed at a respective pistonundercrown portion. In the method, the flow of pressurized oil isreceived in an oil reservoir in the wristpin. In the method, providing ajet from a moving part of each bearing includes providing the jet from awristpin. In the method, providing a jet from a moving part of eachbearing includes sweeping the jet across the respective pistonundercrown portion.

The cooling construction embodiments that are described herein, and thedevices and methods with which they are implemented, are illustrativeand are not intended to be limiting.

1. A piston cooling configuration for an opposed-piston engine,comprising: a piston with crown having an end surface shaped to form acombustion chamber with an end surface of an opposing piston; the crownincluding an undercrown: a bearing mounted in the piston between theundercrown and a small end of a connecting rod; the bearing including awristpin; an oil reservoir in the wristpin in fluid communication withone or more wristpin oil outlet passages positioned to pass oil throughthe wristpin into a lubrication interface between the wristpin and abearing surface; a wristpin oil inlet passage in fluid communicationwith the reservoir, and, at least one cooling jet outlet on one of afixed part of the bearing and a moving part of the bearing; in which thecooling jet outlet is in fluid communication with the oil reservoir andis aimed toward an undercrown portion.
 2. The piston coolingconfiguration of claim 1, in which the wristpin is received on the smallend of the connecting rod.
 3. The piston cooling configuration of claim2, in which: the fixed part of the bearing includes a bearing membersupporting the wristpin for rotatable oscillation with respect to theundercrown in response to movement of the piston; and, the cooling jetoutlet includes an oil outlet passage in the bearing member.
 4. Thepiston cooling configuration of claim 3, in which: the connecting rodincludes an oil passage in fluid communication with the wristpin oilinlet passage; and, the oil outlet passage in the bearing memberreceives pressurized oil from the interface between the wristpin and thebearing surface
 5. The piston cooling configuration of claim 4, in whichthe wristpin and the bearing surface form a rocking bearing.
 6. Thepiston cooling configuration of claim 4, in which the wristpin and thebearing surface form a biaxial bearing.
 7. The piston coolingconfiguration of claim 2, in which: the moving part of the bearingincludes the wristpin, which rotatably oscillates with respect to theundercrown in response to movement of the piston; and, the cooling jetoutlet includes a nozzle mounted to the wristpin.
 8. The piston coolingconfiguration of claim 7, in which the connecting rod includes an oilpassage in fluid communication with the wristpin oil inlet passage. 9.The piston cooling configuration of claim 8, in which the wristpin andthe bearing surface form a rocking bearing.
 10. The piston coolingconfiguration of claim 8, in which the wristpin and the bearing surfaceform a biaxial bearing.
 11. An opposed-piston engine including at leastone cylinder with longitudinally-separated exhaust and intake ports, apair of pistons disposed in opposition to one another in a bore of thecylinder, and a pair of connecting rods, in which each piston is coupledto a respective connecting rod by a bearing, and each piston includes apiston cooling configuration according to any one of claims 1-10.
 12. Amethod for operating an opposed-piston engine including at least onecylinder, a pair of pistons disposed in opposition to one another in abore of the cylinder, and a pair of connecting rods, and in which eachpiston is connected to a respective connecting rod by a bearing, themethod including: providing a flow of pressurized oil to a wristpin ofeach bearing; receiving the flow of pressurized oil in the wristpin;providing multiple streams of the received, pressurized oil from eachwristpin to lubricate a respective bearing interface; and, providing atleast one jet of the received, pressurized oil from one of a fixed partand a moving part of each bearing, each jet being aimed at a respectivepiston undercrown portion.
 13. The method of claim 12, wherein receivingincludes receiving the flow of pressurized oil in an oil reservoir inthe wristpin.
 14. The method of claim 13, wherein providing a jet from amoving part of the bearing includes providing the jet from the wristpin.15. The method of claim 12, wherein providing a jet from a moving partof the bearing Includes sweeping the jet across the respective pistonundercrown portion.